In one type of prior accelerometer, a proof mass is suspended by a flexure hinge between upper and lower excitation rings. The upper surface of the proof mass includes a metal plate that, together with the upper excitation ring, forms a first capacitor. Similarly, the lower surface of the proof mass includes a metal plate that, together with the lower excitation ring, forms a second capacitor. As the proof mass moves upward or downward along its sensitive axis in response to accelerations, movement of the proof mass from its null or centered position is detected by measuring the difference between the capacitances of the first and second capacitors. The resulting "movement" signal causes a servo system to energize electromagnets to return the proof mass to its null position. The magnitude of the drive current provided to the electromagnets is then a measure of the acceleration.
A critical component of such an accelerometer is the pickoff detection circuit used to detect changes in the capacitances of the first and second capacitors. The detection circuit must present the maximum detected signal possible to the servo system in order to minimize the effect of offset voltages and currents. The detection circuit furthermore should operate without placing error sources, such as P-N junctions, in the DC signal path. The problem of sensitive and accurate detection becomes particularly accute when the accelerometer must function in a high radiation environment that imparts parametric changes to the component semiconducting devices. Furthermore, for many applications, the circuit component count must be minimized in order to meet size, weight and packaging constraints.